1. Field of the Invention
This invention relates to a method and an apparatus for drying powdered or granular materials such as synthetic resin materials, processed foot materials, chemical materials for medical use, etc. by submitting them to dielectric heating by means of electromagnetic waves such as microwave, high frequency, etc.
2. Prior Art
Conventionally, a drying method by hot air is widely known as drying method of this kind of powdered or granular materials. As concrete construction of that method, one which is indicated in FIG. 2 is known for example in addition to the so-called hopper dryer provided with a blower, a heater and a hopper.
The system indicated in FIG. 2 consists of a preliminary heating tank A comprising a stirring blade C which agitates the powdered or granular materials supplied through the material inlet B while heating them, a main drying tank F which receives the materials from the preliminary heating tank A through the material inlet G via a rotary feeder E connected to the material outlet D side of the preliminary heating tank A and heats them with the thermal energy of a heater H, a dehumidifying unit J which is provided on the upstream side of the said heater H and supplies dehumidified air, a piping T which returns the exhaust air of the main drying tank F through the exhaust port K to the heating port L of the preliminary heating tank A via a line filter N, a blower O and a heater H.sub.1, and a piping R which returns the exhaust air of the preliminary heating tank A to the inlet Q of the regenerating line of the dehumidifying unit J through the exhaust port P.
Therefore, the powdered or granular materials submitted to preliminary heating in the preliminary heating tank A are dried in the main drying tank F and the powdered or granular materials thus dried to the prescribed moisture percentage are discharged from the material outlet S of the main drying tank F to the subsequent process. The preliminary heating tank A also serves as a crystallizing tank in the case of such powdered or granular materials as non crystallized polyethylene terephthalate, etc.
However, according to the conventional example mentioned above, the dehumidified air from the dehumidifying unit J is heated by the heaters H, H.sub.1 respectively and the heated air is sent by the blower O to the preliminary heating tank A to heat the powdered or granular materials to the desired temperature (and to crystallize them in the case of such powdered or granular materials as non crystallized polyethylene terephthalate, etc.). After that, the heated air is sent to the main drying tank F to dry the powdered or granular materials to the prescribed moisture percentage. In this way, the heated air is made to pass through the tanks A, F as medium.
For that reason, in the conventional example, not only a large amount of electric power was consumed for the heating of air to raise the temperature of powdered or granular materials in the preliminary heating tank A, for the heating of air to regenerate the adsorbent in the dehumidifying unit J and for driving the blower O to send heated air but also the running cost was high because of a lower energy efficiency due to radiation loss in the sending route of heated air such as piping T, piping R, etc.
Moreover, the presence of the line filter N in the middle of the piping T also served to increase the system cost.
In addition, this conventional example was not suitable to such powdered or granular materials as turn yellow by oxidation when heated in the air as nylon.
The present invention is intended to solve all of the problems mentioned above by heating the powdered or granular materials to the prescribed temperature by means of electromagnetic waves such as microwave, etc. under a reduced pressure (and crystallizing them in the case of such powdered or granular materials as non crystallized polyethylene terephthalate, etc.) and, after that, drying them to the prescribed moisture percentage passing through a pressure reducing process.